Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to voltage regulation and, more particularly, to methods and devices for enhancing transient response for voltage regulator circuits.
2. Description of the Background
When precise voltage regulation is required for electronic components, and particularly for microprocessors, it is common to locate a specialized power supply module in close proximity to the voltage sensitive components. This configuration overcomes voltage drops that may be experienced in cables, printed wiring, and/or other connectors that are used to interconnect the power supply module and the voltage sensitive components, and is especially important in cases where the load current changes, thereby causing a change in the voltage drop. Power supply modules used in such applications are commonly referred to as xe2x80x9cpoint of loadxe2x80x9d or xe2x80x9cPOL regulators.xe2x80x9d
A known POL regulator type is a synchronous buck regulator. Synchronous buck topologies are popular because the are very efficient and typically provide an acceptable performance/cost trade-off, especially when the output voltage of the regulator is less than the input voltage, which is a common requirement for many of today""s microprocessors and other low-power electronics.
A buck regulator normally employs an inductor to convert the pulse width modulated (PWM) square waves generated by a control circuit of a synchronous buck converter to an average value. Nevertheless, the output voltage of the regulator may still exhibit a variable or ripple voltage superimposed on the DC output voltage. The magnitude of the ripple is inversely proportional to the inductance of the averaging inductor. Moreover, the response time of the POL regulator is limited by this inductance, as the load current can change at a rate inversely proportional to the inductance. Reducing the value of the inductance to allow the current to increase or decrease more rapidly will increase the ripple voltage. Therefore, there is a minimum value of inductance that can be used.
This situation becomes worse when the load current supplied to the electronic component changes at a fast rate. In these cases the resistance and the inductance of the intermediate connections create both steady-state and transient voltage drops. Normally, filter capacitance is provided at the output of the POL regulator, and current can be supplied or absorbed by these capacitors until the regulator can reach steady-state. Unfortunately, even the best capacitors have internal resistance and inductance that limits their ability to rapidly supply or absorb current.
Furthermore, the feedback control loop of the POL regulator is normally slowed to prevent oscillations. The cumulative effect is that the regulated output voltage can experience a substantial transient change before the regulator recovers. The maximum or minimum voltage during such transient conditions may be outside the limits for the electronic component, thus creating the possibility of a malfunction.
One known manner in which to overcome these drawbacks is to employ multiple buck regulators connected in parallel but operated such that the PWM signals of each regulator are synchronized and shifted in phase. Each buck converter can be designed with a reduced value of inductance, yet due to the overlapping ripple currents, the composite ripple voltage is acceptable. The parallel-connected buck regulators may share the load and have a common feedback loop. Typically two to four parallel-connected buck regulators are used.
The newest microprocessors, however, exhibit such high rates of change in current that even four interleaved buck regulators cannot operate effectively under transient conditions. The response time can be so long that the bypass capacitors substantially discharge to below the minimum required voltage before the POL regulator supplies sufficient current.
Accordingly, there exists a need for a manner in which to enhance the response time of voltage regulator in response to rapid changes in the load current, while at the same time preventing excessive ripple and closed loop instability.
The present invention is directed to a circuit for providing a regulated voltage to a load. According to one embodiment, the circuit includes a power converter coupled to the load and including at least one pulse-width modulated switching device, a control circuit for providing a pulse-width modulated control signal to the pulse-width modulated switching device of the power converter based on an output voltage of the power converter, and a transient override circuit responsive to a load voltage for biasing the pulse-width modulated switching device conductive during certain load voltage conditions.
The voltage regulator circuit of the present invention provides enhanced transient response in comparison to prior art voltage regulator circuits because it decreases the amount of time it takes the circuit to ramp-up the current in the magnetic components of the regulator circuit in response to increased load current. Further, the present invention provides an advantage in that the transient response reaction time has no relation to the crossover frequency of the error signal loop as in prior control methods. As a result, the magnitude of the voltage drop in response to increased load current is reduced with the present invention. At the same time, the present invention permits the error amplifier bandwidth to be rolled off at a lower, more practical frequency, which thereby improves noise immunity and stability margins. In addition, the present invention permits the use of a reduced number of output filter capacitors or lower performance/less expensive output filter capacitors.